This invention relates to processes for converting lower aliphatic alcohol, such as methanol, and olefinic hydrocarbons to high octane liquid fuel. In particular, this invention relates to a system for the production of tertiary-alkyl ethers in the presence of a lower alkanol, such as methanol, combined with the conversion of olefins to gasoline and a transhydrogenation step to convert branched paraffins to branched olefins for recycle.
Technical progress in catalytic olefins upgrading, oxygenate conversion to lower olefins (e.g.- methanol-to-olefins - "MTO"), and the commercial methanol-to-gasoline ("MTG") process have provided important synthetic fuel sources. Also, there has been considerable development of processes synthetic alkyl tertiary-alkyl ethers as octane boosters in place of conventional lead additives in gasoline. The etherification processes for the production of methyl tertiary alkyl ethers, in particular methyl t-butyl ether (MTBE) and t-amyl methyl ether (TAME) have been the focus of considerable research attention to resolve certain limitations in the etherification process with respect to the opportunity to drive the equilibrium dependent etherification reaction to completion by conducting etherification in the presence of excess methanol. It is known that recovering unreacted methanol by conventional separation and extraction techniques imposes severe economic burdens on the etherification process.
Recognizing the common feedstock (e.g. - methanol) for the synthetic production of gasoline as well as the production of methyl tertiary alkyl octane boosting ethers, research workers have endeavored to combine these processes in a manner to provide a synergistically beneficial integrated process.
It is known that isobutylene and other isoalkenes produced by hydrocarbon cracking may be reacted with methanol, ethanol, isopropanol and other lower aliphatic primary and secondary alcohols over an acidic catalyst to provide tertiary ethers. Methanollis considered the most important C.sub.1 -C.sub.4 oxygenate feedstock because of its widespread availability and low cost. Therefore, primary emphasis herein is placed on MTBE and TAME.
In recent years, a major technical challenge presented to the petroleum refining industry has been the requirement to establish alternate processes for manufacturing high octane gasoline in view of the regulated requirement to eliminate lead additives as octane enhancers as well as the development of more efficient, higher compression ratio gasoline engines requiring higher octane fuel. To meet these requirements the industry has developed non-lead octane boosters and has reformulated high octane gasoline to incorporate an increased fraction of aromatics. While these and other approaches will fully meet the technical requirements of regulations requiring elimination of gasoline lead additives and allow the industry to meet the burgeoning market demand for high octane gasoline, the economic impact on the cost of gasoline is significant. Accordingly, workers in the field have intensified their effort to discover new processes to manufacture the gasoline products required by the market place. One important focus of that research is a new process to produce high octane gasolines blended with lower aliphatic alkyl ethers as octane boosters and supplementary fuels. C.sub.5 -C.sub.7 methyl alkyl ethers, especially methyl t-butyl ether (MTBE) and t-amyl methyl ether (TAME) have been found particularly useful for enhancing gasoline octane. Therefore, improvements to the processes related to the production of these ethers are matters of high importance and substantial challenge to research workers in the petroleum refining arts. It is known that isobutylene may be reacted with methanol over an acidic catalyst to provide methyl tertiary butyl ether (MTBE) and iso-amylenes may be reacted with methanol over an acidic catalyst to produce t-amyl methyl ether (TAME). In these etherification processes, a problem of major importance is that methanol is not totally converted and the separation of methanol from the etherification reaction product due to the proclivity of methanol to form a very dilute azeotropic mixture with hydrocarbons and the strong solubility of methanol in both water and hydrocarbons. While it would be beneficial from an equilibrium standpoint to use large excesses of methanol in etherification, subsequent separation problems have limited that process improvement. Due largely to these factors, the cost associated with conventional methanol separation and recycling in the etherification reaction represents approximately 30% of the cost of the total etherification process.
In U.S. Pat. No. 4,684,757 to Avidan et al., the well-known ability of zeolite type catalyst to convert methanol to olefins is utilized by directing unreacted methanol from an etherification reaction to a zeolite catalyzed conversion reaction for conversion to olefin, thereby obviating the need to separate and recycle methanol in the etherification reaction. However, the process incorporates an alkylation step in one embodiment to produce alkylate as well as C.sub.5 + gasoline and C.sub.5 + ethers.
The process for the conversion of methanol to olefins utilized in the Avidan et al. patent is but one in a series of analogous processes based upon the catalytic capabilities of zeolites. It is known that medium pore acid zeolites, such as ZSM-5, can convert methanol to hydrocarbons of higher average molecular weight. Depending on various conditions of space velocity, temperature and pressure methanol, and lower oxygenates in general, can be converted in the presence of zeolite type catalyst to olefins which may then oligomerize to provide gasoline or distillate or may be converted further to produce aromatics.
The feasibility and adaptability of the basic chemistry of zeolite oxygenates conversion to produce useful conversion processes has been the subject of much inventive research activity. Recent developments in zeolite catalyst and hydrocarbon conversion processes have created interest in using oxygenates and olefinic feedstocks for producing C.sub.5 + gasoline, diesel fuel, etc. In addition to the basic work derived from ZSM-5 type zeolite catalyst, a number of discoveries have contributed to the development of a new industrial process. This process has significance as a safe, environmentally acceptable technique for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.5 alkenes. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components into higher hydrocarbons over crystalline zeolites having controlled acidity. Reaction conditions of moderate severity favor the conversion of olefins to predominantly gasoline boiling range products with little paraffins conversion. Milder reaction temperatures and high operating pressures can produce distillate range fuels as well from lower olefins. Garwood et al. have also contributed improved processing techniques in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above identified disclosures are incorporated herein by reference.
A well-known process for the conversion of oxygenates to gasoline is the methanol to gasoline process, known as MTG. The process is described in U.S. Pat. No. 3,931,349 to Kuo, U.S. Pat. No. 4,404,414 to Penick et al. and in the publication by C. D. Chang, Catal. Rev.-Sci. Eng., 25, 1 (1983). These references are incorporated herein in their entirety.
Recognizing the limiting problems of the etherification processes to produce MTBE and TAME and the potential that resides in the general area of the chemistry of oxygenate and olefin conversion with zeolites to resolve those problems, the following objectives of the instant invention have been established: It is an object of the present invention to provide an improved process for the production of high octane gasoline incorporating lower alkyl tertiary-alkyl ethers from isoalkene-rich hydrocarbons, especially MTBE manufacture. It is another object of the present invention to provide an integrated process and reactor system for production of liquid fuels from isoalkene-rich hydrocarbons incorporating etherification with alkanol and olefins conversion and transfer dehydrogenation of paraffins, especially branched C4-C5 isoalkanes.